When kneading concrete materials on site, it is necessary to manage appropriately an amount of water since it has a strong influence on strength of concrete etc. However, a moisture state of the aggregate, which is a concrete material, changes with the storage situations, climatic conditions, and the like. Therefore, if aggregate in wet condition is used, an amount of water in concrete will increase by the amount of surface water of the aggregate. On the other hand, if aggregate in dry condition is used, an amount of water in concrete will decrease by the amount of water absorbed into the aggregate according to an effective absorption of the aggregate.
Therefore, in order to make concrete as specified in a specified mix proportion for maintaining a quality of the concrete, it becomes very important in a case of kneading the concrete to correct the amount of water according to a dryness-and-moisture grade of aggregate.
Referred to as a percentage of surface moisture here is a ratio obtained by dividing mass of the surface water of the aggregate in wet condition (an amount of water adhering to the front face of fine aggregate) by mass of the fine aggregate in the saturated surface-dried condition. Generally aggregate stored, especially fine aggregate is wet in many cases. Therefore, it is common to measure the percentage of surface moisture of the aggregate beforehand as an index of the dryness-and-moisture grade of the aggregate, and to adjust the amount of water based on the percentage of surface moisture measured.
Conventionally, in measurement of such percentage of surface moisture, a small amount of sample has been extracted from a storage container called a stock bin storing fine aggregate for measuring the mass of the sample and the mass of the sample dried completely. Thereafter, the percentage of surface moisture has been computed by using these measured values and the coefficient of water absorption of the aggregate measured beforehand.
However, in such a measuring method, a percentage of surface moisture of aggregate in the stock bin is guessed from a few samples, and therefore accuracy is inferior. On the other hand, in order to measure the mass of aggregate dried completely, heating by a burner or the like is needed. Therefore, it is unreal to measure the mass of aggregate dried completely whose amount is near the actually used amount since much time and expense are needed.
In order to avoid such a problem, an operator checks a kneading situation visually or adjusts the amount of water by using the electric-current value of a mixer. However, these methods have low accuracy primarily. As a result, in order to secure strength of concrete, it is forced to expect a superfluous factor of safety, for example nearly 20%, and it leads to uneconomical mix proportion.